1. Field of the Invention
The present invention relates to a bonded member comprising a ceramic base material and a metallic member which are bonded to each other, and a method for producing the bonded member.
2. Description of the Related Art
As a method of bonding different materials such as a ceramic base material and a metallic member to each other, a method such as one using a solder material can be used. However, during a cooling process after high-temperature bonding, thermal stress caused by a difference in thermal expansion coefficient between the different materials or between the solder material used for bonding the different materials together and the materials occurs, thereby causing separation at the interface between the materials or causing cracks in the vicinity of the interface if one of the materials is brittle, so that desired bonding strength and air tightness cannot be obtained in some cases. Since products having the aforementioned troubles broken during a production process must be discarded as defective products, this results in an unfavorable increase in production costs of these composite bonded members. Further, if the product is subjected to thermal cycles in use, these troubles occur after use for a certain time period to cause a deterioration in the reliability of the product.
As a method of bonding a ceramic base material (hereinafter often simply referred to as “base material”) and a metallic member to each other by use of a solder material, a metallization soldering method and a direct soldering method are available. The metallization soldering method is one in which a metal layer is formed on the surface of a ceramic by deposition from a vapor phase, vapor deposition, sputtering or a method in which paste is applied to the ceramic surface and then heated and then a metallic material is bonded to the ceramic via the metal layer. Meanwhile, as the direct soldering method, an active metal method is particularly well-known in which a metallic material is bonded to ceramic by use of an element in group IV of the Periodic Table as an insert material. Meanwhile, in these methods, unless some measures are taken against thermal stress, which occurs at the interface between bonded materials, so as to reduce the thermal stress, cracks are often formed in a base material which is vulnerable to the thermal stress or separation occurs at the interface, so that bonding strength as well as various properties required from a composite bonded member such as air tightness may be influenced. Particularly, it is very difficult to bond a low-strength base material such as aluminum nitride and a member composed of metal or the like to each other without the occurrence of the above problems.
To solve the above problems, a method of bonding a base material and a metallic member together through liquid-phase bonding using a solder material comprising a metal of low proof stress such as Ag, Cu or Au which undergoes plastic deformation by a low stress can be conceived. However, in a case where a solder material comprising Ag or Cu is used, they may be difficult to use in some cases due to a problem caused by oxidation of the solder material when a composite member obtained is exposed to an application environment of 400° C. or higher under an oxidizing atmosphere, a problem of evaporation of the solder material which is caused by a high vapor pressure when the composite member is used at a high temperature and a low pressure, a problem of migration when the composite member is energized upon use, and a problem of a reaction with an Mo oxide.
Meanwhile, use of a solder material comprising Au is effective for avoiding the foregoing problems which may occur when the Ag or Cu solder material is used. However, when Ni, Co, Kovar or the like is used as a metallic member having excellent oxidation resistance, its constituents (Fe, Ni, Co) diffuse into Au which constitutes the solder material, thereby increasing the proof stress of Au. As a result, the base material is cracked by a thermal cycle and thermal shock.
It is known that when an Au-18Ni solder material and an electrical conductor comprising Mo are bonded to each other, Ni in the solder material reacts with Mo, thereby forming a fragile structure. Accordingly, durability when the bonded interface is exposed to a thermal cycle and thermal shock lowers and the bonded interface deteriorates rapidly, so that there arises a problem that equipment and apparatus having such a bonded interface are liable to be impossible to use.
Meanwhile, use of metal which does not form a solid solution with Au which constitutes the solder material as the metallic member is also conceivable, and illustrative examples of metals which meet the requirement include W and Mo. However, these metals are vigorously oxidized at high temperatures in the air. Therefore, they are not suitable for use as metal materials for members whose exposure to a high temperature atmosphere is imaginable, such as a feeding terminal for a susceptor and a metal ring for attaching the susceptor to a chamber (hereinafter simply referred to as “chamber”) for producing a semiconductor.
To solve the above problems, it has been attempted to devise the bonding structure. For example, JP-A-10-209255 discloses a bonding structure of a ceramic base material and a connector for power supply according to the structure shown in FIG. 4 as a susceptor for disposing a semiconductor wafer. In FIG. 4, a hole 14 is provided in a ceramic base material 1. A metallic member 17 such as Mo which is embedded in the ceramic base material 1 beforehand and has a thermal expansion coefficient approximate to that of the ceramic base material 1 is exposed from the hole 14. Further, a cylindrical atmosphere protector 9 is inserted in the hole 14. Inside the protector 9, a connector 16 for supplying power and a low thermal expansion material 15 intended for stress relaxation are inserted. The protector 9 and the connector 16 are hermetically bonded together by means of a solder material 5, and the material 15 and the protector 9 are hermetically bonded to the metallic member 17 by means of the solder material 5.
According to this bonding structure, residual stresses at the time of bonding are relaxed by the low thermal expansion material 15 and the metallic member 17, and oxidation of the metallic member 17 such as Mo is inhibited by the atmosphere protector 9. Hence, even if bonding is carried out by use of a solder material of high proof stress such as the foregoing Au-18Ni solder material, no cracks occur in the ceramic base material 1, and endurance reliability when the bonded interfaces are exposed to a thermal cycle and thermal shock upon use of a high temperature heater is also high. However, the foregoing bonding structure has such problems that the number of parts is large and that since deterioration of the metallic member 17 occurs due to its oxidation if the atmosphere protector 9 and the metallic member 17 are not fully bonded to each other, a very high production control ability is required.
Further, JP-A-11-278951 discloses, as a susceptor for disposing a semiconductor wafer, a composite member adopting a member structure such as the one shown in FIG. 6 or 7 so as to relax thermal stress which occurs when a corrosion-resistant metal ring 23 made of Kovar or the like is bonded to the back side 22b of a ceramic susceptor 22 in a base material associated with the structure shown in FIG. 5, and a bonding method therefor. Adoption of such a member structure is effective for relaxation of thermal stress. However, since a Cu-based solder material is used, usable temperatures are limited due to the forgoing problem caused by oxidation of the solder material and the foregoing problem of evaporation of the solder material which is caused by a high vapor pressure.
To solve the foregoing various problems, the present inventor has proposed in Japanese Patent Application Number 2000-227291 a composite member produced by solid-phase bonding a ceramic base material and a metallic member to each other by use of an Au solder material, and a production method thereof. According to the production method of the composite member, since solid-phase bonding makes it possible to bond the base material and the metallic member together with low proof stress of the Au solder material maintained, the base material is hardly broken at the time of bonding, and durability of the interface between the bonded materials hardly lowers even when the interface is exposed to a thermal cycle or thermal stress. Thus, the production method exhibits such significantly excellent effects. However, in the solid-phase bonding adopted in the production method of a composite member which is proposed in the foregoing specification, a pressing operation and other operations are required, and it is imaginable that procedures of the operations may be complicated in some cases. Hence, the method is susceptible to further improvements.